Marine and land-based industrial (M & I) gas turbine engines are frequently derived from engines designed for and used in various types of aircraft. Such M & I engines are used, for example, for powering marine vessels, electrical generators, and various types of pumping and compression applications.
The parent gas turbine engine of an M & I engine is typically designed and constructed to be lightweight and to operate at minimum specific fuel consumption (SFC) in an aircraft for predetermined thermodynamic cycles of operation having predetermined ranges of air and combustion gas flowrates, temperatures, and pressures in the engine. The cycles are also preselected for maximizing thermal and propulsive efficiency of the engine.
Development of an aircraft gas turbine engine requires a substantial amount of design, development, and testing resulting in substantial development costs. In designing gas turbine engines for M & I applications, it has proved to be more cost effective to modify an existing aircraft gas turbine engine in the desired power class, than to design the M & I engine from the beginning. Accordingly, it is desirable to minimize the changes in the aircraft engine required for obtaining a suitable M & I engine.
One application of an M & I engine is to provide peaking power for powering an electrical generator to provide additional electrical power to a utility power grid when the utility power demands exceed on-line base load capacity. For example, the parent aircraft gas turbine engine may be adapted specifically for driving an electrical generator at a synchronous speed such as 3,000 rpm or 3,600 rpm, for generating electricity at 50 hertz or 60 hertz, respectively. The utility industry desires relatively simple and inexpensive gas turbine engines which can be brought on-line quickly and then shut down quickly as required to meet the peaking requirement. It is also desired to generate the required power during peaking operation as efficiently as possible for reducing kilowatt-hour cost.
One factor in obtaining relatively low kilowatt-hour cost is the development cost for providing an industrial gas turbine engine for meeting the required power demands. In order to keep development costs relatively low, the industrial gas turbine engine typically utilizes a parent aircraft gas turbine engine and makes as few changes as practical in the design thereof. Furthermore, obtaining maximum output shaft horsepower from the engine used for providing peaking power is desirable for maximizing the amount of electrical power generated by the generator for also reducing costs.
An industrial gas turbine engine derived from a dual rotor aircraft turbofan engine for directly driving an electrical generator presents new problems. In a conventional dual rotor aircraft turbofan engine, the two rotors are free to rotate substantially independently of each other and thus provide for improved stall margins of the booster compressor. In the industrial engine, the low pressure turbine (LPT) rotor is required to run at the synchronous generator speed and, therefore, its conventional use in controlling engine operation is no longer available. And, operating flexibility of the engine is narrowed for preventing any undesirable decrease in stall margin of the compressors.
Accordingly, the core engine, or gas generator, speed and the booster compressor variable inlet guide vanes (VIGVs) are the primary means for controlling output shaft horsepower to the generator. In order to obtain maximum possible output shaft horsepower for peaking power applications, the gas generator is operated at a maximum core speed for providing a maximum amount of energy to be extracted by the LPT. The thermodynamic cycle of the engine, therefore, requires maximum core airflow and maximum operating temperatures for obtaining maximum output shaft horsepower from the LPT.
However, operating the engine at maximum output shaft horsepower, also provides a maximum temperature of the high pressure compressor (HPC) discharge air in the core engine. The HPC discharge air is typically used for cooling relatively hot turbine components of the engine, and therefore, the temperature thereof affects the useful life of the cooled components. Whereas increased operating temperatures of the engine are desired for maximizing output shaft horsepower, decreased temperature of the HPC discharge air is desired for obtaining acceptable life of the turbine components.
An engine is typically sized and rated at standard conditions, such as, for example, at sea level at 59.degree. F. (15.degree. C.). At such standard conditions, the maximum output shaft horsepower of the engine may be provided, i.e. rated, which is the maximum value obtainable from that particularly sized engine for obtaining a predetermined useful life thereof.
For example, one dual rotor preexisting aircraft turbofan gas turbine engine has a sea level standard conditions (e.g. 59.degree. F. (15.degree. C.)) rating of about 60,000 pounds (27,216 kilograms) of thrust. In adapting this aircraft turbine engine for powering an electrical generator, the engine derived therefrom has a rating of about 56,000 output shaft horsepower (SHP) at sea level standard conditions (59.degree. F. (15.degree. C.)). However, this particular engine is limited in operation by the temperature (i.e. T.sub.3) of the high pressure compressor discharge air. The HPC discharge air temperature T.sub.3 has a maximum value; i.e., T.sub.3.sup.max, at about the 59.degree. F. (15.degree. C.) rating point for obtaining a predetermined useful life of the turbine components thereof. Accordingly, on hot days where the ambient temperature increases beyond about 59.degree. F. (15.degree. C.) up to about 119.degree. F. (48.3.degree. C.), the engine must be operated at output shaft horsepower values correspondingly less than the maximum value at 59.degree. F. (15.degree. C.) for preventing the HPC discharge air temperature T.sub.3 from exceeding its maximum permissible value T.sub.3.sup.max. At an ambient temperature of about 119.degree. F. (48.3.degree. C.), the maximum rated amount of output shaft horsepower from this engine is about 38,000 SHP.